Injectors are devices that expel fluid, such as radiopaque media (contrast fluid) used to enhance x-ray or magnetic images, from a syringe, through a tube, and into an animal subject. Injectors are typically provided with an injector unit, adjustably fixed to a stand or support, having a plunger drive that couples to the plunger of the syringe and may move the plunger forward to expel fluid into the tube, or move the plunger rearward to draw fluid into the syringe to fill it.
Injectors often include control circuits for controlling the plunger drive so as to control the rate of injection and amount of fluid injected into the subject. Typically, the control circuit includes one or more manual switches which allow a user to manually actuate the plunger drive to move the plunger into or out of the syringe; typically the user holds down a "forward" or "reverse" drive switch to move the plunger in the indicated direction.
To reduce the risk of infection, in a typical injection procedure the syringe is only used once, and is disposed after use. In some cases, the syringe is inserted into the injector empty. The empty syringe is filled by retraction of the plunger while the interior of the syringe communicates with a supply of the contrast fluid via an injection tube connected between the nozzle of the syringe and the supply of media. Then, bubbles are removed from the syringe, and the injection is performed. At the end of the procedure, the syringe plunger typically is forward, as is the plunger drive.
In some injectors, the syringe can only be removed or replaced while the plunger drive is fully retracted. As illustrated in FIG. 1A, typically an empty syringe 10 is filled with sterile air, with the plunger 12 at the fully retracted position as shown. The plunger drive includes a jaw 18 designed to engage and disengage a button 14 on the rear side of the plunger while the plunger is in this fully-retracted position. Before an empty new syringe can be filled, it is necessary that the plunger be moved fully forward in the syringe so that the syringe can be filled by rearward retraction of the plunger. Thus, the reloading operation can involve fully retracting the plunger drive to allow removal and replacement of the syringe, then fully advancing the plunger drive and plunger to expel air from the syringe, and then retracting the plunger drive and plunger to fill the syringe. These lengthy, manual movements of the plunger and drive are time consuming.
The above-referenced patent application describes a front-loading injector in which a syringe can be replaced even though the plunger drive is not fully retracted. This injector substantially reduces the number of plunger drive movements necessary to prepare a syringe for a new injection; after an injection, the syringe can be removed and replaced without moving the drive from its fully-advanced position. (The plunger drive jaw 18 can engage and disengage button 14 regardless of the position of the plunger.) After the syringe is replaced, the drive is retracted, filling the syringe for a new injection. Thus, to ready the injector for a new injection, the plunger drive is manually moved once rather than three times.
Another recent development is the use of pre-filled disposable syringes. A pre-filled syringe also reduces the number of manual plunger drive movements necessary to prepare the injector for a new injection. After an injection, the plunger drive is fully retracted, the used syringe is removed and replaced with the pre-filled syringe, and the injector is ready for a new injection. Thus, again, the plunger drive is manually moved once rather than three times.
To prevent infection, contrast media remaining in a syringe after an injection must be discarded. However, contrast media is relatively expensive. For this reason, when preparing for an injection, an empty syringe is filled with only as much media as will be needed for the next injection. For the same reason, pre-filled syringes are sold in a number of capacities, e.g. ranging from 60 to 125 milliliters, allowing the operator preparing for an injection to select a syringe containing only as much media as is needed for the injection.
A typical pre-filled syringe is illustrated in FIG. 1B. In many respects, the pre-filled syringe is identical to the empty syringe shown in FIG. 1A. The barrels 10 and plungers 12 have the same size and profile in both syringes (injectors now in use accommodate only a few FDA approved syringe sizes, e.g., a 200 milliliter size and a 125 milliliter size, so all syringes use these sizes). Furthermore, both syringes have a button 14 which is initially located at the end of the barrel 10 (thus, both syringes are compatible with injectors which are designed to grip a button at the end of the syringe). The main difference is that in the pre-filled syringe of FIG. 1B, the initial location of the plunger 12 is in the middle of the syringe (thus reducing the initial volume of the pre-filled syringe). An extender 16 is attached to button 14 of the plunger, and provides a second button 18 at the end of the syringe which can be gripped by the injector.